VCSEL arrays can be used as low-cost, high-power light sources for a variety of military and commercial applications. Currently, the cost of high power (500 to 1000 W) Nd:YAG and CO2 lasers exceed $100/Watt. Coherent VCSEL arrays have been realized in the premises network market where low power (<0.005 W), VCSEL based transceivers selling for ˜$100 dominate the market, having displaced edge-emitting semiconductor laser based transceivers that sell in the $500 to $5000 range.
Other applications, such as optical pumps at 980 nm for Erbium-doped fiber amplifiers (EDFAs) for telecommunications will benefit from low cost lasers with increased power (0.05 to 1.0 W). High-powered EDFAs cannot be achieved from single element VCSELs, but could be economically fabricated with VCSEL arrays. VCSEL arrays are theoretically capable of kW of power with current material and heat sinking technology.
Aside from these commercial and economic applications, coherent arrays of VCSELs have far-reaching significance because they have the potential to deliver >>1 W of power, over a wide variety of wavelengths. These devices can be used to accelerate progress in areas such as medicine, communications, manufacturing, and national defense.
High-power VCSEL arrays have been demonstrated by several research groups. Grabherr et al. reported VCSEL power densities exceeding 300 W/cm 2 from a 23-element array [M. Grabherr et. al., Electron. Lett., vol. 34, p. 1227, 1998]. Francis et al. demonstrated VCSEL power in excess of 2-W continuous wave and 5 W pulsed from a 1000-element VCSEL array [D. Francis, et. al., IEEE Int. Semiconductor Laser Conf. (ISLC), Nara, Japan, October 1998]. Chen et al. also reported the power density of 10 kW/cm 2 from an array of 1600 VCSELs using a microlens array to individually collimate light from each laser [H. Chen, et. al., IEEE Photon. Technol. Lett., vol. 11, No. 5, p. 506, May 1999]. However, their beam quality at high power is still poor. A high quality beam requires a narrow linewidth single mode with high spatial and temporal coherence.
In order to produce coherent, single-frequency, high-power arrays of VCSELs, the elements of one or two-dimensional VCSEL arrays should be phase-locked. Although the light from each individual VCSEL is coherent, the phase and frequencies (or wavelengths) of the light from each VCSEL are slightly different, and therefore uncorrelated. For such an incoherent array consisting of N elements producing the same power P, the on-axis power in the far-field is ˜NP. However, if the array can be made coherent, in phase, and with a single frequency, the on-axis power in the far-field is N2P and the width of the radiation pattern is reduced by ˜1/N. This high on-axis far-field power is required in laser applications such as free space optical communications and laser radar where a large amount of power is required at a distance, or in applications such as laser welding, laser machining, and optical fiber coupling that require high power to be focused to a small spot.
Previous efforts to phase-lock arrays of VCSELs have used diffraction coupling [J. R. Legar, et. al., Appl. Phys. Lett., vol. 52, p. 1771, 1988] and evanescent coupling [H. J. Yoo, et. al., Appl. Phys. Lett., vol. 56, p. 1198, 1990]. Diffraction coupling depends on geometrical scattering of light and evanescent coupling requires that the optical field of adjacent array elements overlap. Both approaches impose restrictions on the array architecture. More importantly, these existing approaches have had very limited success, even in 1D edge-emitting arrays where both approaches have been extensively investigated. Recently, Choquette et al. has demonstrated phase locking in a VCSEL array using an anti-guide approach [D. K. Serkland, et. al., IEEE LEOS Summer Topical Meeting, p. 267, 1999].